Light-Triggered Alarm
When a beam of light hits the sensor, the buzzer screams.
What you’ll learn
- Using an LDR as a switch input.
- Threshold-based triggering — the 555 fires only above a certain light level.
- How a small sensor change drives a big audible reaction.
What you need
| Item✓ | Qty | ~Cost | Where to buy | |
|---|---|---|---|---|
| 9V battery + snap clip | 9V battery + snap clip | 1 | GHS 23.00 | Any supermarket or hardware store. |
| 220 Ω resistor | 220 Ω resistor Look for red-red-brown stripes. | 1 | GHS 0.75 | Electronics shop or online (Amazon, Adafruit). Look for red-red-brown stripes. |
| Red 5 mm LED | Red 5 mm LED Any colour works; red is brightest. | 1 | GHS 1.50 | Electronics shop or online. Any colour works; red is brightest. |
| Half-size breadboard | Half-size breadboard | 1 | GHS 23.00 | Online (Amazon, Adafruit, AliExpress). |
| Jumper wires | Jumper wires | 4 | GHS 30.00 | Online — usually sold in packs of 30. |
| 555 timer IC (NE555 / TLC555) | 555 timer IC (NE555 / TLC555) Bargain even by the bag of 10. | 1 | GHS 7.50 | Electronics shop or online. Bargain even by the bag of 10. |
| 10 kΩ resistor | 10 kΩ resistor Brown-black-orange. | 1 | GHS 0.75 | Electronics shop or online. Brown-black-orange. |
| 100 kΩ resistor | 100 kΩ resistor Brown-black-yellow. | 1 | GHS 0.75 | Electronics shop or online. Brown-black-yellow. |
| 10 µF capacitor (electrolytic) | 10 µF capacitor (electrolytic) | 1 | GHS 1.50 | Electronics shop or online. |
| 100 nF capacitor (ceramic) | 100 nF capacitor (ceramic) | 1 | GHS 0.75 | Electronics shop or online. |
| LDR / photoresistor | LDR / photoresistor | 1 | GHS 7.50 | Online. |
| Piezo buzzer (passive) | Piezo buzzer (passive) | 1 | GHS 15.00 | Online. |
| TotalTotal | ~GHS 112.00 | |||
Build it step by step
Astable 555 from Project 6
Same circuit, audio-rate (R = 10 k + 47 k, C = 100 nF). Output drives the buzzer.
Hook the LDR to pin 4
LDR between + rail and pin 4 (reset). 47 kΩ between pin 4 and −. In the dark, pin 4 stays low → chip is held in reset → silent.
Shine a flashlight on the LDR
Resistance drops, pin 4 rises above ~1 V, the 555 starts oscillating, the buzzer screams.
How it works
Pin 4 of the 555 is the "reset" pin — when it’s pulled low, the chip is forced silent regardless of what the rest of the circuit is doing. We use the LDR + 47 kΩ as a voltage divider that keeps pin 4 low in the dark and high in the light. Cross the threshold, the chip wakes up, the buzzer beeps.
If something’s not working
Buzzer never goes off
- · LDR is wired in parallel with the divider, not in series.
- · Pin 4 isn’t pulled to + when the LDR is lit — try a 22 kΩ pull-down instead.
Buzzer screams all the time
- · Pin 4 is wired straight to + — there’s nothing to gate it.
Try this next
Flip the divider so the alarm fires in the dark instead.
Add an LED in parallel with the buzzer — visual + audible alarm.